The present invention relates to a cooling fan controlling apparatus for controlling an electric cooling fan used with a radiator in an automotive vehicle.
In a conventional down-flow type cooling system of an engine, coolant from the radiator and the coolant returned from a heater line are supplied to a coolant inlet of the engine by a pump. The coolant heated by the engine is returned to the radiator from the upper portion of the engine. A line interconnecting the radiator and the coolant inlet is opened and closed by a thermostat.
The open/closed operation of the thermostat is dependent upon a temperature of the returning coolant from the heater line. At an outside air temperature in winter or in a cold region which requires the use of a heater, the in the heater coolant cooled through heat exchange with indoor blowing air contacts the thermostat so that the temperature of the thermostat becomes lower than a set value, thereby causing a low frequency of opening of the thermostat.
In contrast, at ordinary outside air temperatures not using the heater or at a high outside air temperature, the coolant heated in the engine does not pass the heater without the heat exchange with the indoor blowing air, and contact the thermostat, so that the temperature of the thermostat becomes higher than the set value, thereby causing a high frequency of opening of the thermostat.
The frequency of opening of the thermostat is less at the low outside air temperature than at the ordinary outside air temperature or at high temperature. Accordingly, the amount of the coolant supplied from the radiator to the engine is reduced at the low outside air temperature, whereas the temperature at the coolant outlet of the engine becomes relatively high.
The cooling fan for the radiator begins operation when the coolant temperature reaches a predetermined set value, the coolant temperature being detected by a coolant temperature sensor which is mounted on a line near the coolant outlet of the engine because the engine temperature is estimated from the coolant temperature.
Therefore, the frequency of operations of the cooling fan becomes high at low outside air temperatures, and there arises a problem of high noise level.
The cooling effect of the cooling fan changes with the outside air temperature even if the same amount of inside blowing air is used. It is obvious that the coolant temperature decreases faster at the low outside air temperature than at high outside air temperatures even if the cooling fan is driven at the same rotation speed.
Therefore, at low outside air temperatures with a high frequency of operations of the cooling fan, there is no danger to engine overheating due to a rise of the coolant temperature. In such a case, unnecessary energy loss is generated if the cooling fan is driven at the same set temperature as the ordinary outside air temperature or high temperature having a relatively low frequency of operations of the cooling fan.
In order to solve these problems, there is proposed in Japanese Patent Laid-open Publication No. 58-192917 a coolant temperature switch using a wax and having a plurality of contacts wherein one of the power supply lines to the cooling fan motor is selected in accordance with an outside air temperature to change the coolant temperature at which the cooling fan motor is operated.
With this prior art, however, it is necessary to use an outside air temperature detector. In addition, the coolant temperature for initiating the cooling fan motor is changed by selecting the contact of the coolant temperature switch in accordance with the outside air temperature. Therefore, if the coolant temperature is required to be set with precision, the number of contacts of the coolant temperature switch becomes large, resulting in a complicated structure and low reliability. Further, the precise control is not attained because the coolant temperature switch is the mechanical switch using the wax.